Rigid foams having polyisocyanurate structures are known. They are highly cross-linked polymers and therefore have high compressive strength, high temperature resistance and high dimensional stability to heat. They also have excellent flame-resistant properties. The possible uses of polyisocyanurate foams therefore lie primarily in the field of thermal insulation. When provided with facings, they may also be used in the building industry as structural elements.
The production of polyisocyanurate foams is known and has been described, for example, in German Pat. No. 1,112,285 and in British Pat. No. 1,104,394, and may be carried out, for example, by reacting organic polyisocyanates with known trimerization catalysts in the presence of materials, such as foam stabilizers and blowing agents, and, optionally, polyols. The polyisocyanate used is frequently crude diisocyanatodiphenylmethane of the type obtained from a commercial phosgenation of crude diaminodiphenylmethane (see, e.g. British Pat. No. 1,146,661 and 1,184,893).
The wide field of application potentially available to these foams by virtue of their high temperature resistance combined with high flame resistant properties is, however, severely restricted by their brittleness. Attempts have therefore been made to reduce this brittleness.
In British Pat. No. 1,318,925, for example, it has been proposed to achieve this object by using polyester polyols within a preferred molecular weight range of from 1500 to 3000. The use of an increased proportion of polyfunctional propylene oxide polyethers has also been investigated, for example, by Frisch et al in Journal of Cellular Plastics, Sequence 1, Number 6 (1970), pages 203-214. In U.S. Pat. No. 3,849,349, the simultaneous use of higher functional polyols having a preferred molecular weight range of from 100 to 300 and epoxides has been proposed.
In another process for reducing the brittleness of polyisocyanurate foams it was proposed to use prepolymers having isocyanate end groups obtained from trifunctional polyesters and tolylene diisocyanate, such as the prepolymers described by Nicholas and Gmitter in Journal of Cellular Plastics, Volume 1, Number 1 (1965), page 85, and the prepolymers described in U.S. Pat. No. 2,979,485 and 2,993,870 and in German Offenlegungsschrift 2,024,344. However, the foams obtainable by this type of process (for example, those described in German Offenlegungsschrift No. 1,745,177) were and are still regarded as too brittle for practical use.
Yet another process has been proposed according to which branched polyols which have hydroxyl numbers within the preferred range of from 400 to 600 and (commercially) pure diisocyanatodiphenylmethane are reacted together in proportions, by weight, of less than 0.5. Various examples of the advantageous use of a higher proportion of difunctional isocyanates for the production of polyisocyanurate foams have also been given in German Offenlegungsschriften No. 1,966,261 and No. 1,769,023.
Since the combination of high dimensional stability to heat with high non-inflammability is highly desirable, these known methods for producing polyisocyanurate foams, while achieving a certain reduction in brittleness, do so only to the extent that the foams are not destroyed in transport or when installed or exposed to the usual mechanical influences in use. These polyisocyanurate foams, even though less brittle, must therefore still be classified as brittle, rigid foams.
Additionally, it has not hitherto been known to produce polyisocyanurate foams which are thermoformable. The process of thermoforming flexible and semi-rigid polyurethane foams is known and has been extended to rigid foams, for example according to the teaching given in German Gebrauchsmuster No. 7,220,186. These foams are preferably produced from difunctional polyols having hydroxyl numbers within the range of from 150 to 300 by reaction with polyisocyanates and may contain a proportion of isocyanurate structures. The index is within the conventional range for rigid polyurethane foams (from 100 to 110). Even this process gives no guidance as to how systems which have higher indices can be converted into thermoformable polyisocyanurate foams by trimerization of excess polyisocyanate.
In view of the convenience and advantages of the thermoforming process, there has long been a desire to produce rigid foams which would combine the advantageous properties of polyisocyanurate foams with the capacity for being thermoformed. Such foams have not hitherto been known.